The present invention relates to an optical amplifier with a doped fluoride glass fibre and a process for the production of said amplifier. It has particular applications in the field of optical fibre telecommunications.
Wavelength multiplexing is a promising method for increasing the transmission capacity of an optical fibre, without its scope being limited by the chromatic dispersion of the fibre, or for giving flexibility to the transmission or distribution network.
The fibre optical amplifier has become a key component of optical telecommunications and the use of a multiplex of wavelengths requires that the optical amplifier has specific performance characteristics. It must be able to amplify the different wavelengths of the multiplex with the same gain, whilst subjecting said wavelengths to an identical noise.
Erbium-doped, silica fibre amplifiers have a non-flat spectral gain and the transmission of a multiplex of wavelengths across an erbium-doped silica fibre amplifier can only take place if the wavelengths are in a flat part of the gain spectrum of said amplifier, which only represents part of the available amplification band of such an amplifier.
Various methods have been proposed for flattening or equalizing the gain spectrum of said erbium-doped, silica fibre optical amplifiers and in particular the use of an optical filter placed in the centre of the amplifying fibre, the use of a fibre laser loop and the use of a not completely pumped fibre.
These methods make it possible to effectively flatten the gain spectrum of said amplifiers, but reduce said gain.
It was established some years ago that by doping with erbium the core of an optical fibre based on fluoride glass, such as a fluorozirconate glass (such as e.g. ZBLAN), the gain spectrum of an amplifier using this type of fibre was naturally flat, i.e. the gain, as a function of the wavelength, was virtually constant in a natural manner for wavelengths close to 1.5 .mu.m, so that such an amplifier is a candidate for multiwavelength transmission.
However, experience acquired with multiwavelength transmission with an erbium-doped, fluoride glass fibre shows that there is a reduction in the gain in that part of the optical spectrum which corresponds to the lower wavelengths, which limits the pass band of the amplifier.
In the centre of the gain spectrum, there is also a reduction of approximately 1.5 dB, which limits the performance characteristics of a "cascade" of erbium-doped, fluoride glass fibre amplifiers. It is therefore necessary to flatten the gain spectrum of erbium-doped, fluoride fibre-based amplifiers.